Anthropogenic global change is altering the ways that forests function, but it can be immensely challenging to put hard numbers on the magnitude of those changes. For instance, whilst nobody would dispute that introduced deer and rodents are having immediate effects on New Zealand's native forests (e.g. browse damage is apparent in most forest understories), quite what the long term effects will be remains unclear. The difficulty is that these introduced animals affect only the establishment phase of the life cycle, and it takes many decades, even centuries, for them to have noticeable effects on the forest canopy. Process-based modelling provides a way of quantifying long-term impacts.